Also, docker has very convenient UX. You can download, configure & run any container with just one command. Compare with virtualbox or qemu. You can say that docker is like flatpak for servers.
Plus, its isolation helps against dumb bugs in shitty software you just wanted to run once, e.g. [1]. I'm not talking about security here, only dumb bugs.
[1]: https://github.com/ValveSoftware/steam-for-linux/issues/3671